Application Of Hierarchical Aluminophosphate Molecular Sieves In The Oxidation Of Aromatic Hydrocarbons

Hierarchical aluminophosphate molecular sieves show good catalytic performance in the field of heterogeneous catalysis due to its regular pore structure and good thermal stability.Doping metal atoms into the framework of Al PO-5 molecular sieve,isomorphously replacing Al or P atoms,can introduce active centers for molecular sieve and increase acidic sites.The hierarchical molecular sieve is composed of two or more grades of pore structure,which can overcome the limitation of single pore channel,reduce the mass transfer resistance,and expand the application field of the hierarchical molecular sieve on the basis of maintaining the catalytic activity and stability of the molecular sieve.This paper uses hydrothermal method and dual template to synthesize hierarchical heteroatom aluminophosphate molecular sieve D-MeAPO-5,and analyzes the physical and chemical properties of molecular sieve by XRD,SEM,N₂ adsorption-desorption,NH₃-TPD and other characterization methods.Furthermore,the catalytic performance of D-MeAPO-5 on the oxidation of aromatic hydrocarbons was studied.First,D-MeAPO-5 Molecular sieve is synthesized by using pseudoboehmite as the aluminum source,phosphoric acid as the phosphorus source,triethylamine and cetyltrimethylammonium bromide as the dual template,and doped with different metal atoms.It was found that the morphology of synthesized D-MeAPO-5 molecular sieve is columnar,with good crystallinity,owning weak and medium-strong acid sites.The catalytic performance of D-MeAPO-5 molecular sieves was compared in benzene oxidation reaction.The catalytic performance of D-VAPO-5 molecular sieve was better,benzene conversion was 24.97%,and phenol yield and selectivity were 17.34%and 69.48%,respectively.Four factors affecting phenol yield（reaction temperature,reaction time,catalyst dosage,oxidant dosage）were optimized by using the CCD design in response surface methodology.The quadratic polynomial mathematical model of phenol yield was established.The actual value was close to the predicted value of the model,and the relative error was 1.04%.Then,using V₂O₅ and NH₄VO₃ as vanadium sources,and the molar ratio of gel is 1.0 Al₂O₃:1.0 H₃PO₄:1.0 TEA:0.05 V₂O₅:0.45 CTAB:100 H₂O,D-VAPO-5 molecular sieve was synthesized with or without fluoride ions.The experimental results show that the valence state of vanadium changes from trivalent to pentavalent after calcination of the synthesized vanadium-doped hierarchical aluminophosphate molecular sieve with V₂O₅ or NH₄VO₃ as the source of vanadium.The molecular sieve synthesized under fluorine-free conditions with V₂O₅ as the vanadium source did not detect vanadium in the SEM-EDS analysis.And the total acid content measured by NH₃-TPD is relatively small,0.85 mmol/g.When NH₄VO₃ is used as the vanadium source and fluorine is present,the total acid content of the synthesized molecular sieve is relatively large,which is 1.57 mmol/g.The synthesized D-VAPO-5molecular sieve is used as catalyst for toluene oxidation.The catalytic effect of the molecular sieve synthesized under the condition of fluorine with V₂O₅ as the vanadium source is better.The conversion rate of toluene reached 21.0%,and the yields of benzaldehyde,benzyl alcohol,and cresol were 9.1%,2.4%and 5.5%,respectively.Finally,density functional theory（DFT）is used to study the adsorption process of VAPO-5 molecular sieve and H₂O₂,and the Gibbs free energy and reaction energy barrier of the reaction are calculated.The geometric configurations of reactants,products,intermediates and transition states are optimized and vibrationally analyzed on the B3LYP/6-31G level.Use the intrinsic reaction coordinate（IRC）to verify the correctness of the transition state.The possible path for VAPO-5 to catalyze the benzene oxidation reaction is speculated:the first step is VAPO-5 adsorbs H₂O₂ to generate H₂O,the second step is the transfer of H atoms in the framework after the adsorbed complex removes H₂O molecules,and the third step is molecular sieveAdsorption of benzene directly generates phenol.